An assembly of the type specified above is known from WO 2011/067057, for example. Described therein is the provision of a photopolymer formulation which comprises three-dimensionally crosslinked polyurethane matrix polymers, a writing monomer and a photoinitiator. This formulation is then applied to a substrate and cured, and forms a photopolymer film. Substrates proposed include polyethylene terephthalate foils.
A disadvantage of the known assembly, however, is the very strong adhesion between substrate and photopolymer film. Parting the photopolymer film from the substrate is therefore possible only using high peel forces, in which case often there may be damage to the photopolymer film. Non-destructive detachment of the exposed photopolymer film from the substrate, however, is critical to its integration into devices such as security documents or display installations.
One theoretical solution to this problem would be to increase the thickness of the photopolymer film, since such films would have an enhanced capacity to withstand the high peel forces. For the majority of photopolymer film applications, however, the need or wish is for the lowest possible layer thickness, and so in the great majority of instances this is not a feasible route.
Alternatively, attempts could be made to reduce the adhesion between substrate and photopolymer film. That could be done using substrates with low surface tension. In that case, however, new problems arise, since producing a photopolymer film with sufficient surface consistency is possible only by the addition to the photopolymer formulation of substantial quantities of highly surface tension-lowering additives. When such quantities of additives of this kind are employed, however, the result in many cases is the clouding of the formulation. In the worst-case scenario, indeed, there is separation of the components. If, on the other hand, smaller amounts of the additives are used, only partial wetting of the substrate with the photopolymer formulation is achieved. The result then is, in turn, photopolymer films which incompletely cover the substrate. Depending on the surface tension of the substrate, however, it is no longer possible to achieve uninterrupted coating in the desired coat thicknesses, even when using large amounts of surface additives. Formulations of these kinds, therefore, cannot be used to obtain useful photopolymer films.